A three-year-old female spayed boxer presented to the Emergency Service at Cornell University Hospital for Animals (CUHA) in respiratory distress. The patient had a history of treatment with antibiotics for a suspected pneumonia, with minimal improvement, for approximately three weeks. On presentation, initial blood work revealed a mild thrombocytopenia, leukocytosis and marked eosinophilia. Thoracic radiographs revealed severe, asymmetric, bilateral lung consolidation with a pulmonary mass and tracheobronchial lymphadenopathy. The patient was hospitalized on supplemental oxygen therapy, antibiotics, and intravenous fluids. Overnight she became acutely painful and non-weight bearing in her hind limbs. Her distal extremities were cold with absent femoral pulses. Based on the diagnosis of a suspected distal aortic thrombus, euthanasia was elected. Gross necropsy confirmed an aortic thrombus and multiple nodules were seen in the lungs and spleen. Histopathology revealed severe eosinophilic infiltration in multiple organs including lungs, spleen, liver, and lymph nodes. An increased M:E ratio was noted in the bone marrow. Based on clinical and histopathologic findings, a diagnosis of idiopathic hypereosinophilic syndrome was made.